I see where you’re coming from but I have to disagree. You don’t have to do physical activities like sword training or magic to mentally grow. In my opinion he isn’t childish but more naive. He’s grown healthy and happily in a safe environment where he wasn’t forced to grow or mature fast compared to the FL. Of course the FL will seem more mature this is her second life, she has more experiences, and has grown up in a much more harsh environment. This is one of the few stories I’ve read where the position between ML and FL are switched. If the roles were reversed you wouldn’t be saying that FL isn’t a match for the ML because she’s immature. I just think us as readers are too used to having a mature, cold, older, nonchalant ML and anything opposite of this is seen as “childish or immature”.
I feel like this is 90% the ML dad's fault. He's child-like himself. I think the manga trope of 'all physical contact of or speaking to the opposite gender = total system failure due to embarrassment' is on full display here.
Also, "trust servants 100% because my brother trusted them 0%" is straight up idiotic.
Wanna be assassinated...?
Cuz that's how you get assassinated
True you don't have to learn magic or swordsmanship to be considered mature mentally. However, he calls the duke his master, but we don't see him ever training; it's like hand-to-hand combat -- you're training your mind to anticipate and defend against an opponent who has a different thinking concept from you. (Kinda like chess.) And when you’re learning from/with a teacher you’re not only picking up their techniques you're absorbing their life lessons that help you grow. And I'm just not seeing it in him when I read.
And I do appreciate that the ML’s upbringing was full of love, value, and appreciation. (Take note authors: not everyone has to come from an abusive household/family life.) And, like you stated, I wonder if it was his naivety and innocence that was rubbing me the wrong way. As I couldn’t see the ML as nothing but a big kid and nowhere close to reaching adulthood. Perhaps, their country is more laid-back, because even there I don’t see any growth (good or bad) that would show him becoming one-day the future ruler of a country. I wouldn’t have minded if he was shown a little stoic in front of outsiders/other nobles, but relaxed (goofy) when he was back home. A mental switch.
I did say I wouldn’t compare the ML and FL’s lives as their experiences were different. I don’t expect him to be hardcore and bloodthirsty – that’s not his environment. But I did expect/want him to be less childlike to me. His upbringing was full of love and care, but did that stunt his mental growth?? Allowing him to remain too childlike?? (My perception.)
And this is where I really disagree: naive/innocent FLs. Being female myself, I am way more harder on them and their portrayals. (Probably much more harsher than they deserve.) I have dropped many a story because I couldn’t stand an FL character. I don’t expect all my FLs to be badass, kickass, stone-cold killers, but I do expect them to exhibit a modicum of maturity and sense to match their situations. (Plus, not to repeatedly make dumb decisions!!) Since I hold FLs to higher standard than MLs, I often either outright drop the series or bookmark it for a long pause until I feel like I can pick it back up to either finishing reading it or to drop it from my reading list.
I agree with your viewpoints and statements. I think you’re right that’s he’s too immature and naive and hasn’t had any development. But, I think he’ll grow out of it. He’s still in his late teens so he has time to mature and develop. Hopefully the story will focus on him and not just have him as a simple love interest for the FL. Also I apologise for misjudging you and saying you wouldn’t judge FL’s for the same thing. That was incorrect on my part. I think I got too carried away. On this website I usually see people bash the nice and kind MLs and hype the abusive and redflag ML’s.
No apologies needed. You helped me with your comments to examine my real reaction to the ML and not just a post-chapter reaction to what I just read.
And since I haven't dropped this story, I'm hoping, like you, that the ML will lose the naivety, while he continues to grow and mature through the story. (Note: I don't want ML's growth to come with a huge devastating life change.)
I have no love for abusive or rapist red/black flags MLs. They NEVER deserve an HEA!! (That's probably why I read more action/adventure stories than romance on here.)

I'm about to get some hate with this comment, but I can't keep in any longer: I can't stand the ML. I don't think he's a good partner for the FL. The ML doesn't feel like he had any growth in the years they have been apart. He doesn't match the FL's emotional maturity. He gives off a childlike vibe that doesn't resonate with me. Being childlike when they first met, fine, okay, even though he's several years older than her, but in the subsequent years apart he hasn't grown -- only in height. We don't see him do any training (sword or magic) that will help facilitate his mental growth. Yes, the FL is more mature due her circumstances, but the ML is the Crown Prince and needs the mental wherewithal to handle that position. Right now they feel too mixed match to form any adult relationship. And, no, I didn't want him to turn into an aloof ML but I would hope the author would tune down that goofball side of him or at least give him more than physical maturity to balance it out.